The present invention relates to an air cushion conveying device with magnetic guidance. It more particularly applies to the conveying of silicon wafers in an ultra-clean environment.
Silicon wafers are loaded into cassettes. The displacement of these wafers during the transfer between cassettes or between a cassette and a processing machine (etching, deposition, etc.) must be carried out in such a way that polluting dust is avoided to the greatest possible extent.
Various means are used for the conveying operation. A first system consists of two cylindrical, parallel belts, each guided by two rollers or pulleys. The rotation of the latter makes it possible to convey a wafer located on the belts. However, the said system suffers from the disadvantage that the belts give off particles produced by their rubbing against the rollers. The particles can stick to the silicon wafers and pollute them.
A second known system consists of an articulated arm provided with a "sucker" finger. The wafer to be conveyed is kept by suction on the "sucker" finger. This device causes little pollution, but only permits short-distance paths and is expensive.
Compared with the first two systems, preference is generally given to the known system of levitation on an air cushion. Such a system is diagrammatically shown in FIG. 1. A silicon wafer 10 is carried and moved by an air cushion above a track 12. The latter has two longitudinal, parallel pipes 14 issuing obliquely through orifices 16 on its upper face facing the wafer 10. Pressurized air is blown by a not shown blower system into the pipes 14, so as to pass out again through the orifices 16 as oblique jets.
However, the latter device still suffers from disadvantages. If the wafer 10 is not guided, it can pass out of the track 12. To avoid this, lateral guides 18 are added on either side of the track 12.
However, in order to be guided, every so often the wafer strikes a lateral guide 18, so that polluting particles are released. The moving wafer has a certain kinetic energy. In order to stop it, use is made of a system of abutments 20 located at the end of the track 12 which are associated with a vacuum system which, by suction, fixes the wafer 10 to track 12. The vacuum system consists of a pipe 15 arranged longitudinally in track 12 and issuing onto the latter through an orifice 17. Pipe 15 is connected to a not shown suction system. During the stopping by abutments 20, as hereinbefore described, the impact produces polluting particles. The jets discharged through the orifices 16 form a powerful air flow to the rear of wafer 10, which sometimes disengages particles. These particles can then be stuck to the wafer. This phenomenon destroys the surface state of the wafer.